Carbon sequestration: pulling off an ecological hat-trick?

Dr Assise Touré

Dr Assise Touré explains how remote sensing and computer simulations are helping West Africa to take advantage of the Clean Development Mechanism.

Climate change presents perhaps the direst of threats to our world and the global economy. Without radical changes to our way of living and the development of clever new strategies to counter carbon accumulation in the atmosphere, the world is on course for more extreme climate events such as floods and droughts. In dry areas such as the Sahel, to this can be added severe land degradation, and irreversible desertification.

Carbon sequestration is one such clever new strategy. Carbon exists in five different locations – or ‘pools’ – on the planet: in the atmosphere, the oceans, the terrestrial biomass (plants and animals), geological formations and soils. Carbon sequestration is the facilitated transfer of carbon from the air to other pools, thus reducing the amount of atmospheric CO2. This transfer, or ‘flux’, sees natural processes of photosynthesis take carbon from the air and transform it into organic material that enters the soil via plant roots, organic litter and crop residues. In this way, soils become a carbon ‘sink’.

The process could achieve an ‘ecological hat-trick’. Not only could it ameliorate carbon accumulation in soils, but it would also help to combat desertification, improve soil fertility, and enhance biological diversity. Better yet, as the Clean Development Mechanism (CDM) of the Kyoto Protocol allows developing countries to sell carbon credits to developed nations to offset carbon emissions, sequestration would also deliver substantial economic benefits to farmers and other stakeholders in Africa.

However, before such projects can be launched in a region, local stakeholders must first estimate its sequestration potential. This requires quantifying the dynamics of carbon sources and sinks over space and time, and modelling the major processes that control CO2 exchange between the soil and the atmosphere.

The SOCSOM project

Sequestration of Carbon in Soil Organic Matter (SOCSOM) is a project launched by USAID to investigate this potential in Senegal. The project used remotely sensed imagery and biogeochemical computer modelling to develop carbon sequestration strategies that could eventually be applied across the country.

The semi-arid and sub-humid regions of Africa have the highest potential for carbon sequestration in the world due to their severely degraded soils. But by increasing irrigation, reduced tillage, restoring grasslands and savannas, and extending fallow periods, coupled with applications of compost and organic waste and the adoption of erosion control strategies, countries could replenish their soil carbon stocks. In the process, this would benefit farmers through increased agricultural productivity.

In a pilot study in southern Senegal, SOCSOM researchers employed remote sensing to depict the trends and sensitivity of carbon stocks under various management and climate scenarios over a 200-year period from 1900 to 2100. They estimated the patterns of carbon stocks and fluxes in 1900 by setting up the General Ensemble Biogeochemical Modelling System (GEMS), a computer simulation, to run for 1500 years, based on actual vegetation, soil, drainage and climate data from 1961 to 1996.

Using a detailed timeline of remotely sensed imagery from the Landsat satellite taken in 1973, 1978, 1984, 1990 and 1999, and field-checked in 1984, 1996 and 2001, they were able to map the changes in land and carbon status across the country. The system was then used to simulate the impact of human activities on carbon dynamics by incorporating information on changes in land cover and land use. Additionally, data on crop composition, crop rotation patterns, grazing, fire, fertilizer use and irrigation from the literature and censuses were incorporated into the simulations. Finally, various possible future climate change scenarios were layered into the model.

Carbon credit payments

The model results showed the actual and potential soil carbon gains, and the economic and ecological costs and benefits. Between 1900 and 2000 the total carbon stock in soils and vegetation went from 141 to 89 tonnes per hectare – a reduction of 37 per cent.

That may sound bad, and it is, but in the world of carbon sequestration, this is actually great news, because it means that Senegal has that much more ‘space’ left to fill with carbon. And what does this signify financially to the country in terms of carbon credit payments from developed nations? Some €70 million a year, or just under €2.5 billion over the 35 years that it will take to reach a new carbon steady-state.

In order for the carbon sequestration process to be maintained over that period, national capacities for monitoring and simulating soil carbon stocks using remote sensing, GIS and biogeochemical modelling must be improved. Thus, after the original SOCSOM pilot projects were completed, the effort to develop sequestration strategies for West Africa was split into two complementary programmes. The first, SEMSOC, is developing locally integrated project activities with partners in Burkina Faso, Ghana, Mali and Niger, to achieve a ‘Sahel and West Africa-wide synthesis’ of the region’s carbon sequestration potential. The second programme, SOCSAB, based at the AGRHYMET Centre in Niger, focuses on training in the use of modelling technologies.

All of which brings great hope to the struggle against global warming and not insignificant financial benefits to Senegalese farmers.

Dr Assise Touré ( assize@cse.sn ) is the technical director of the Centre de Suivi Ecologique in Senegal. For further information visit the website .

11 March 2006

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